1. Field of the Invention
The present invention relates to a laser drawing apparatus for drawing a pattern on a surface of a workpiece by scanning the workpiece surface with laser beams, then by modulating the laser beams on the basis of raster-graphic data in accordance with a series of clock pulses.
2. Description of the Related Art
Such a laser drawing apparatus is used for drawing a fine pattern on a surface of a suitable workpiece. As a representative use of the laser drawing apparatus, a fine circuit pattern may be laser drawn on the workpiece when producing a printed circuit board using photolithography. In this case, the workpiece may be either a photosensitive film for producing a photomask or a photoresist layer formed on a suitable substrate.
Recently, a circuit pattern drawing system has been developed for efficiently carrying out a drawing of circuit pattern by integrating the use of a laser drawing apparatus, a CAD (Computer Aided Design) station for designing a circuit pattern, a CAM (Computer Aided Manufacturing) station for editing the designed circuit, an EWC (engineering work station) for commanding and controlling the drawing operation of the individual laser drawing apparatus and, finally, a LAN (local area network) through which the CAD station, the CAM station, the EWS and the laser drawing apparatuses are all connected to each other.
A circuit pattern is, thereby, designed and treated as vector-graphic data by the CAD station. The vector-graphic data may be fed from the CAD station to the CAM station, through the LAN, for editing of the designed circuit pattern, if necessary. In either case, the vector-graphic data is fed from the CAD station or CAM station to the EWS, through the LAN, and is stored in a recording medium, such as a hard disk, provided in the EWS.
The EWS suitably feeds the vector-graphic data to one of the laser drawing apparatuses, and the vector-graphic data are converted into raster-graphic data by a vector-to-raster graphic converter provided in the laser drawing apparatus. The raster-graphic data is developed and stored in a pattern memory (or bit-map memory) and, subsequently, the raster-graphic data is successively read from the pattern memory. A circuit pattern is then drawn on a surface of a workpiece by scanning the workpiece surface with the laser beams and by modulating the laser beam on the basis of the read raster-graphic data in accordance with a series of clock pulses.
Workpieces are inevitably subjected to fine dimensional variations or fluctuations derived from production-factors and environmental conditions under which the workpieces are produced. Accordingly, before proper dimensions can be given to a circuit pattern which is to be drawn on each of the workpieces, a scaling-correction must be carried out during a drawing-operation of the laser drawing apparatus.
For example, when a workpiece is two-dimensionally expanded with respect to it's standard dimensions thereof, the scaling-correction must be carried out such that a circuit pattern to be drawn on the expanded workpiece is, also, two-dimensionally scaled-up in accordance with dimensional differences between the expanded dimensions and the standard dimensions. On the contrary, when a workpiece is contracted with respect to the standard dimensions, the scaling-correction must be carried out such that a circuit pattern to be drawn on the contracted workpiece is scaled-down in accordance with dimensional differences between the contracted dimensions and the standard dimensions.
Conventionally, the scaling-correction has been statistically carried out under the assumption that workpieces manufactured under the same production-requirements are subjected to equal degrees of dimensional variations or fluctuations. Nevertheless, in actuality, there may be relatively large dimensional variations or fluctuations among the workpieces manufactured under the same production-requirements. Accordingly, it cannot be said that the conventional statistical scaling-correction is necessarily reliable.
Further, conventionally, the scaling-correction is introduced into vector-graphic data in the CAM station. In this case, the scaling-correction is merely performed with a unit of a one-pixel size with respect to a circuit pattern to be drawn. Namely, in the conventional laser drawing apparatus, it is impossible to perform a precise scaling-correction with a unit of less than the one-pixel size.